Intravascular catheters are used in a wide variety of minimally invasive medical procedures. Several types of catheters are utilized for intravascular treatment. Examples of intravascular catheters include guide catheters, angioplasty catheters, stent delivery devices, angiography catheters, neuro-catheters, and the like. Such intravascular catheters may be used for diagnostic or therapeutic purposes. Generally, an intravascular catheter enables a physician to remotely perform a medical procedure by inserting the catheter into the vascular system of a patient at a location that is easily accessible and thereafter navigating the catheter to the desired target site. Using such procedures, virtually any target site in the patient's vascular system may be remotely accessed, including the coronary, cerebral, and peripheral vasculature.
In order to function efficiently, many intravascular catheters require a relatively stiff main body portion and soft distal portion and tip. The stiff main body portion gives the intravascular catheter sufficient “pushability” and “torqueability” to allow it to be inserted, moved and rotated in the vasculature to position the distal end of the catheter at the desired site adjacent to a particular vessel. However, the distal portion should have sufficient flexibility so that it can track over a guidewire and be maneuvered through a tortuous path to the treatment site. In addition, a soft distal tip at the very distal end of the catheter should be used to minimize the risk of causing trauma to a blood vessel while the intravascular catheter is being moved through the vasculature to the proper position. Thus, variable stiffness catheters, having a relatively stiff proximal portion and a relatively flexible distal portion are desirable. Variable stiffness catheters are achieved by varying the properties of the materials used to manufacture the catheters.
One difficulty which has arisen for meeting demands for greater neurovascular catheter length is that the diameter of the distal section necessarily becomes smaller, since the longer catheters must reach ever narrower vascular areas. This smaller distal portion diameter requires a concomitant thinning of the wall section of the more distal portions of the catheter. The thinner distal section walls are able to attain even higher flexibility, which is a desirable trait because of the higher level of tortuosity in distal vasculature. In known methods of manufacturing catheters, those thinner walls have lower column strength and are more prone to kinking or rippling when actively pushed along the guidewire or when vaso-occlusive devices are pushed through the catheter's lumen. Furthermore, the different stiffness sections are matted creating joints which may not be sufficiently robust, or they may overlap and/or have sharp transition areas between the sections, which can increase susceptibility to kinking.
One known method of manufacturing catheters intended for use as angiography catheters or as guiding catheters often comprise a tubular liner surrounded by an outer tubular shell, with a reinforcing layer interposed there between. Either the outer shell or the liner, or both tubular elements may include relatively softer polymeric materials in a distal region of the catheter. Optionally, the reinforcing layer (e.g., tubular braid) may also have a more flexible, modified form in the distal region.
Another known method of manufacturing variable stiffness catheters is to produce a laminated catheter assembly with a uniform polymeric material. Selected regions of the catheter are then modified by radiation treatment to selectively increase stiffness. However, having a composite construction of a catheter using different materials is preferable and with this method of manufacturing, the choice of materials, as well as having control of the final catheter properties, is limited.
Another known method of manufacturing variable stiffness catheters requires sliding a series of tubular segments having different stiffness over (and onto) an inner assembly comprising a liner surrounded by a reinforcing layer. The tubular segments are shrink-fitted and melt-bonded to the inner assembly using a removable length of heat-shrink tubing. Such a process is labor intensive and inefficient since it requires many different materials for each segment and the catheters can only be fabricated one-at-a-time.
Yet another method of manufacturing catheters requires a continuous extrusion of a first rigid polymer to form an inner tubular body. Then, extruding a second soft, pliable polymer over (onto) the rigid tubular body to form an outer layer. Additionally, the catheter may be reinforced with a stiffening material, typically a wire cord or a braid wrapped around or embedded within the layers of the catheter. However, the distal section of the catheter may not be soft enough or the proximal section may not be stiff enough by limiting the materials to just one type of inner rigid layer and one type of soft pliable polymer, making this type of catheter unsuitable for passage through tortuous vasculature.
In another manufacturing method known as reel-to-reel process, an outer jacket material is varied by switching between extrusion sources as a continuous length of inner assembly passes through a wire-coating type extruder head. Alternatively, discrete sections of one material are extruded or over-molded onto the continuous length of inner assembly. A different material is then extruded onto the length of inner assembly, filling in the spaces between the discrete sections. After forming the continuous, variable stiffness outer shell, the long assembly is cut into catheter length sections. Although, the reel-to-reel method is a more cost efficient than assembling catheters one-at-a-time, the use of different materials to achieve variable catheter stiffness requires multiple assembly steps and/or complex tooling, and the junctions between the different material sections require careful control of design and manufacturing to avoid potentially weak joints that could fail during use.
Another method of manufacturing variable stiffness catheters include cutting segments of multi-layer tubular members and joining them together end to end, with the distal segment having a reduced durometer and/or thickness compared to its adjacent more proximal segment. However, the joints created by the mating of segments of tubular members may not be sufficiently robust to sustain tensile strength and other reliability requirements. This is because most of the tubular members for catheters are multi-layers extrusions having an innermost layer made of polytetrafluoroethylene (PTFE) that is particularly difficult to join end-to-end and is typically not melt-bond compatible with a nylon or polyether block amide (Pebax®) outermost layer. Butt-join or lap-join multi-layer tubular member segments have been unreliable because all abutting or overlapping layers tend not to successfully bond to one another.
Further approaches to improve joint reliability and overall manufacturability have common drawbacks, such as the tubing needed to be heated along its entire length to bond the various pieces together, and an entire length of shrink tubing covering the length of the tubing must be used to bond the layers and then discarded. Depending on the length of the variable stiffness catheter, the extended shrink tubing amounts to a considerable overall cost increase (multiple extrusions, shrink tubing, more direct labor required for assembly) relative to a conventional multi-layer extrusion, making the cost essentially prohibitive for highly segmented variable stiffness catheters.
The above described methods of manufacturing, particularly, the current fused extrusion methods are laborious and time intensive. Significant amounts of hand work and reliance on long lead items such as extruded tubing makes the process of design iteration time consuming and skill dependent. In addition, the current methods of manufacturing lack some design flexibility, especially in the area of material stiffness transitions. Thus, more graded transitions are desirable but are not used in complex designs requiring complex transitions. Furthermore, in many cases there is a need to have an inner and outer layer of a catheter shaft made of different materials, and although, this can be achieved with co-extruded tubing or multiple layers of laminated tubing, the design flexibility is limited in view of required specialized tubing builds and long lead times.